The present application(s) claim(s) priority to Japanese Application(s) No(s). P2000-261396 filed Aug. 30, 2000, which application(s) is/are incorporated herein by reference to the extent permitted by law.
1. Field of the Invention
This invention relates to a method of growing a polycrystalline silicon layer, a method of epitaxially growing a single crystal silicon layer, and a catalytic CVD apparatus, which are suitable for manufacturing, for example, a thin-film transistor (TFT).
2. Description of the Related Art
For fabricating a polycrystalline silicon (Si) layer, typically used heretofore was a method using atmospheric pressure chemical vapor deposition (APCVD) to decompose silane (SiH4) or disilane (Si2H6) under a temperature around 600 to 700xc2x0 C., in hydrogen atmosphere and under the pressure of 1xc3x97105 Pa (760 Torr) and thereby grow the layer, a method using low-pressure chemical vapor deposition (LPCVD) to decompose and grow silane (SiH4) or disilane (Si2H6) under a temperature around 600 to 700xc2x0 C., in hydrogen atmosphere and under the pressure of (0.53xcx9c1.33)xc3x97102 Pa (0.4xcx9c1 Torr) and thereby grow the layer, or a method using plasma CVD to decompose silane (SiH4) or disilane (Si2H6) under a temperature around 200 to 400xc2x0 C., in a hydrogen atmosphere and under the pressure of (0.26xcx9c2.6)xc3x97102 Pa (0.2xcx9c2 Torr), thereby grow an amorphous silicon layer and thereafter anneal the amorphous silicon layer under a high temperature around 800 to 1300xc2x0 C. so as to grow crystal grains.
However, those methods for growing polycrystalline silicon layers by APCVD and LPCVD involve the problem that their growth temperatures are high. In APCVD and LPCVD, since all of the energy required for chemical interaction and growth during growth of polycrystalline silicon layers is supplied in form of heat energy by heating, the growth temperature cannot be largely decreased from about 600xc2x0 C. Additionally, since interaction efficiency of reactant gas like silane is generally as low as several t or less, almost all of such reactant gas is discharged and discarded, cost of reactant gas becomes high and cost required for the discard is also high. On the other hand, the method for fabricating a polycrystalline silicon layer by crystallizing an amorphous silicon layer involves the problem that it additionally needs an annealing apparatus for high-temperature annealing.
Recently, as a growth method of polycrystalline silicon layers overcoming those problems, a growth method called catalytic CVD are being remarked (for example, Japanese Laid-Open Publication No. sho 63-40314, Japanese Patent Laid-Open Publication No. hei 8-250438, Japanese Patent Laid-Open Publication No. hei 10-83988 and Applied Physics, Vol. 66, No. 10, p. 1094(1997)). This catalytic CVD uses catalytic cracking reaction between a heated catalyst and reactant gas (source material gas). Catalytic CVD, in its first stage, brings reactant gas (such as silane and hydrogen in case of using silane as the source material of silicon) into contact with a hot catalyst heated to 1600 through 1800xc2x0 C., for example, to activate the reactant gas and thereby make silicon atoms, or clusters of silicon atoms, and hydrogen atoms, or clusters of hydrogen atoms, having high energies, and in its second stage, raises the temperature of these silicon atoms and hydrogen atoms or molecules having high energies, or a substrate that supplies their clusters, to a high temperature, thereby to supply and support the energy required particularly for silicon atoms to form single-crystal grains. Therefore, catalytic CVD enables growth of a polycrystalline silicon layer even at a lower temperature than those of conventional APCVD and LPCVD, such as around 350xc2x0 C., for example.
However, according to results of various experiments made by the Inventor, in the case where a polycrystalline silicon layer is grown at a low temperature by existing catalytic CVD, metal impurities more easily enter into the growth layer than in growth layers grown by conventional APCVD and LPCVD, and containment of high-concentrated metal impurities in the resultant polycrystalline silicon layer is a problem this technique involves. These contained metal impurities amount to, for example, 2xc3x971017xcx9c2xc3x971018 atoms/cc, of tungsten (W), 7xc3x971015xcx9c2xc3x971017 atoms/cc of iron (Fe), 9xc3x971014xcx9c3xc3x971016 atoms/cc of chromium (Cr), and less than 3xc3x971018 atoms/cc of nickel(Ni). In contrast, concentrations of metal impurities contained in a polycrystalline silicon layer grown by conventional APCVD or LPCVD are less than 1xc3x971015 atoms/cc of W, typically around 5xc3x971016 atoms/cc of Fe, typically less than 3xc3x971014 atoms/cc of Cr, and typically less than 6xc3x971019 atoms/cc of Ni. Thus it is recognized how high the concentration of metal impurities contained in the polycrystalline silicon layer grown at a low temperature by catalytic CVD. Polycrystalline silicon layers containing metal impurities to a high concentration exhibit bad electric properties, such as having a low electron mobility, and when they are used as polycrystalline silicon layers for TFT, for example, it is difficult to operate the TFT at a high speed.
It is therefore an object of the invention to provide a polycrystalline silicon layer growth method that can grow a polycrystalline silicon layer remarkably low in concentration of metal impurities contained therein.
Another object of the invention is to provide a single crystal layer epitaxial growth method that can epitaxially grow a single crystal silicon layer remarkably low in concentration of metal impurities contained therein.
Still another object of the invention is to provide a catalytic CVD apparatus that can grow a polycrystalline silicon layer and a single crystal silicon layer remarkably low in concentration of metal impurities contained therein.
The Inventor made researches toward solution of the above problems involved in conventional techniques. These researches are outlined below.
According to the Inventor""s researches, it is considered that W, Fe, Cr and Ni contained in polycrystalline silicon layers grown by conventional catalytic CVD derived exclusively from catalysts. Among them, W is the component element of a catalyst itself whereas Fe, Cr and Ni are considered to have been contained as impurities in the W material. W has a very high melting point as high as 3380xc2x0 C. and a low vapor pressure, its amount taken into the growth layer is not considered to be so much. Actually, however, since oxidizing substances like O2 and H2O exist in the growth chamber of the catalytic CVD apparatus, W forming the catalyst will be oxidized to tungsten oxide when the catalyst is heated to a high, and tungsten oxide having a high vapor pressure will vaporize and will be taken into the growth layer.
Through various experiments, the Inventor reached the conclusion that, in order to prevent or minimize ingestion of metal impurities into a growth layer, it would be most effective to form a barrier layer on the surface of the catalyst to prevent separation of disengagement of component elements or impurities from the catalyst when it is heated to a high temperature for growth and that a carbide or a carbide would be preferable as the barrier layer from the viewpoint of heat resistance and easiness of its formation. It is sufficient for the barrier layer to exist on the surface of the catalyst at least upon the start of growth. It may be previously formed before placing the catalyst in the catalytic CVD apparatus, or may be formed before the growth is started after the catalyst is placed in the catalytic CVD apparatus.
The present invention has been made as a result of researches based on the Inventor""s own knowledge.
According to the first aspect of the invention, there is provided a polycrystalline silicon layer growth method for growing a polycrystalline silicon layer on a substrate by catalytic CVD, characterized in:
the polycrystalline silicon layer being grown by using a catalyst having a nitride that forms at least the surface thereof.
According to the second aspect of the invention, there is provided a single crystal silicon layer epitaxial growth method for epitaxially growing a single crystal silicon layer on a material layer in lattice alignment with the single crystal by catalytic CVD, characterized in:
the polycrystalline silicon layer being epitaxially grown by using a catalyst having a nitride that forms at least the surface thereof.
According to the third aspect of the invention, there is provided a catalytic CVD apparatus using a catalyst, characterized in:
said catalyst having a nitride at least on the surface thereof.
In the first, second and third aspects of the invention, the nitride on the surface of the catalyst may be thick enough to prevent component elements or impurities from separating or disengaging externally at the temperature for using the catalyst. More specifically, a thickness not smaller than 1 nm is sufficient as the thickness of the nitride although it depends on the adhesiveness of the nitride with its base and the film quality of the nitride as well. For more reliable prevention of external separation of component elements or impurities from the catalyst, thickness of the nitride is preferably 5 nm or more, or more preferably not less than 10 nm. The nitride is typically made by nitrifying the surface of the catalyst before conducting the growth. Nitrification is normally conducted by heating the catalyst in an atmosphere of a gas containing nitrogen. In case a catalyst of tungsten, for example, is used, since the tungsten nitride formed on the surface of the catalyst of tungsten at a high temperature may suffer local cracks or exfoliation when the temperature of the catalyst decreases, for the purpose of preventing tungsten from oxidization and vaporization from cracks or portions of exfoliation, tungsten nitride is preferably formed on the surface of the catalyst immediately before the growth of the silicon layer. In case the surface of a catalyst of tungsten, for example, is nitrified, if nitrification is conducted at a temperature between 400xc2x0 C. and 770xc2x0 C., there occur disadvantages including an increase of the resistance value caused by nitrification of not only the surface of the catalyst but also the entirety. Therefore, the catalyst of tungsten is preferably heated in a gas atmosphere containing nitrogen to a temperature in the range from 800xc2x0 C. to 2200xc2x0 C., more preferably in the range from 800xc2x0 C. to 2200xc2x0 C., or more preferably in the range from 1600xc2x0 C. to 2100xc2x0 C. or in the range from 1700xc2x0 C. to 1900xc2x0 C. In these temperature ranges, a good-quality nitride can be formed at a practical speed. The gas containing nitrogen may be, for example, ammonium (NH3), nitrogen (N2) or hydrazine (N2H4). Upon heating the catalyst to the temperature for its use or the nitrification temperature, the catalyst is desirably held in a hydrogen atmosphere for the purpose of preventing oxidization by oxidizing components existing in the atmosphere. On the other hand, in case the catalyst of tungsten is nitrified after raising the temperature to the range from 1700xc2x0 C. to 1900xc2x0 C., since a slight amount of oxidizing components existing in the growth chamber may oxidize and evaporate tungsten and may stack tungsten on the substrate surface for growing the silicon layer on, for the purpose of preventing it, nitrification is conducted by first heating the catalyst of tungsten to a first temperature in the range from 800xc2x0 C. to 1600xc2x0 C. in a hydrogen atmosphere, for example, and thereafter heating the catalyst of tungsten to a second temperature in the range from 900xc2x0 C. to 2200xc2x0 C. in a gas atmosphere containing nitrogen. More preferably, the first temperature is in the range from 900xc2x0 C. to 1100xc2x0 C. whereas the second temperature is in the range from 1200xc2x0 C. to 2200xc2x0 C. More preferably, the first temperature is in the range from 900xc2x0 C. to 1100xc2x0 C. (typically around 1000xc2x0 C.) whereas the second temperature is in the range from 1600xc2x0 C. to 2100xc2x0 C.
By growing a polycrystalline silicon layer or a single crystal silicon layer by catalytic CVD using a catalyst of tungsten having formed a nitride on the surface thereof, for example, the maximum tungsten concentration in the polycrystalline silicon layer or the single crystal silicon layer can be controlled to 5xc3x971016 atoms/cm3 or less.
The nitride may be formed by stacking a nitride on the surface of the catalyst, instead of forming it by nitrifying the surface of the catalyst. For stacking the nitride, sputtering, metal-organic chemical vapor deposition (MOCVD), or other like method, for example, can be used.
On the other hand, in order to reduce the concentration of contained oxygen in addition to the concentration of the contained metal impurities in the polycrystalline silicon layer or single crystal silicon layer grown by catalytic CVD, it is effective to adjust the total pressure of the growth atmosphere to a value in the range from 1.33xc3x9710xe2x88x923 Pa to 4 Pa at least at the beginning of the growth, or adjust the partial pressure of oxygen and moisture in the growth atmosphere to a value in the range from 6.65xc3x9710xe2x88x9210 Pa to 2xc3x9710xe2x88x926 Pa at least at the beginning of the growth. By adjusting the total pressure of the growth atmosphere or the partial pressure of oxygen and moisture to a value in the above-indicated ranges, ingestion of oxygen to the growth layer can be reduced significantly. As a result, in case of a polycrystalline silicon layer, the maximum oxygen concentration of a portion at least 10 nm deep from the boundary between the substrate and the polycrystalline silicon layer can be reduced to a value not higher than 5xc3x971018 atoms/cm3, or 2.5xc3x971018 atoms/cm3 in some cases. Alternatively, the maximum oxygen concentration of a portion at least 50 nm deep from the boundary between the substrate and the polycrystalline silicon layer can be reduced to a value not higher than 5xc3x971018 atoms/cm3. Alternatively, the maximum oxygen concentration of a portion at least 100 nm deep from the boundary between the substrate and the polycrystalline silicon layer can be reduced to a value not higher than 5xc3x971018 atoms/cm3. In case a single crystal silicon layer, the maximum oxygen concentration of a portion at least 10 nm deep from the boundary between the material layer and the single crystal silicon layer can be reduced to a value not higher than 3xc3x971018 atoms/cm3, or 2xc3x971018 atoms/cm3 in some cases. Alternatively, the maximum oxygen concentration of a portion at least 50 nm deep from the boundary between the material layer and the single crystal silicon layer can be reduced to a value not higher than 3xc3x971018 atoms/cm3. Alternatively, the maximum oxygen concentration of a portion at least 100 nm deep from the boundary between the material layer and the polycrystalline silicon layer can be reduced to a value not higher than 3xc3x971018 atoms/cm3. If the thickness of the single crystal silicon layer is 1 xcexcm or less, the maximum oxygen concentration can be reduced to a value not higher than 3xc3x971018 atoms/cm3. If the thickness of the single crystal silicon layer is 100 nm or less, then the maximum oxygen concentration can be reduced to a value not higher than 2xc3x971018 atoms/cm3.
According to the fourth aspect of the invention, there is provided a polycrystalline silicon layer growth method for growing a polycrystalline silicon layer on a substrate by catalytic CVD, characterized in:
the polycrystalline silicon layer being grown by using a catalyst having a carbide that forms at least the surface thereof.
According to the fifth aspect of the invention, there is provided a single crystal silicon layer epitaxial growth method for epitaxially growing a single crystal silicon layer on a material layer in lattice alignment with the single crystal by catalytic CVD, characterized in:
the polycrystalline silicon layer being epitaxially grown by using a catalyst having a carbide that forms at least the surface thereof.
According to the sixth aspect of the invention, there is provided a catalytic CVD apparatus using a catalyst, characterized in:
the catalyst having a carbide at least on the surface thereof.
In the fourth, fifth and sixth aspects of the invention, the carbide on the surface of the catalyst may be thick enough to prevent component elements or impurities from separating or disengaging externally at the temperature for using the catalyst. More specifically, a thickness not smaller than 1 nm is sufficient as the thickness of the carbide although it depends on the adhesiveness of the carbide with its base and the film quality of the carbide as well. For more reliable prevention of external separation of component elements or impurities from the catalyst, thickness of the carbide is preferably 5 nm or more, or more preferably not less than 10 nm. The carbide is typically made by carbonizing the surface of the catalyst before conducting the growth. Carbonization is normally conducted by heating the catalyst in an atmosphere of a gas containing carbon. In case a catalyst of tungsten, for example, is used, since the tungsten carbide formed on the surface of the catalyst of tungsten at a high temperature may suffer local cracks or exfoliation when the temperature of the catalyst decreases, for the purpose of preventing tungsten from oxidization and vaporization from cracks or portions of exfoliation, tungsten carbide is preferably formed on the surface of the catalyst immediately before the growth of the silicon layer. In case the surface of a catalyst of tungsten, for example, is carbonized, if carbonization is conducted at a temperature between 400xc2x0 C. and 770xc2x0 C., there occur disadvantages including an increase of the resistance value caused by carbonization of not only the surface of the catalyst but also the entirety. Therefore, the catalyst of tungsten is preferably heated in a gas atmosphere containing carbon to a temperature in the range from 800xc2x0 C. to 2200xc2x0 C., more preferably in the range from 800xc2x0 C. to 2200xc2x0 C., or more preferably in the range from 1600xc2x0 C. to 2100xc2x0 C. or in the range from 1700xc2x0 C. to 1900xc2x0 C. In these temperature ranges, a good-quality carbide can be formed at a practical speed. The gas containing carbon may be, for example, methane (NH4). Upon heating the catalyst to the temperature for its use or the nitrification temperature, the catalyst is desirably held in a hydrogen atmosphere for the purpose of preventing oxidization by oxidizing components existing in the atmosphere. On the other hand, in case the catalyst of tungsten is carbonized after raising the temperature to the range from 1700xc2x0 C. to 1900xc2x0 C., since a slight amount of oxidizing components existing in the growth chamber may oxidize and evaporate tungsten and may stack tungsten on the substrate surface for growing the silicon layer on, for the purpose of preventing it, carbonization is conducted by first heating the catalyst of tungsten to a first temperature in the range from 800xc2x0 C. to 1600xc2x0 C. in a hydrogen atmosphere, for example, and thereafter heating the catalyst of tungsten to a second temperature in the range from 900xc2x0 C. to 2200xc2x0 C. in a gas atmosphere containing carbon. More preferably, the first temperature is in the range from 900xc2x0 C. to 1100xc2x0 C. whereas the second temperature is in the range from 1200xc2x0 C. to 2200xc2x0 C. More preferably, the first temperature is in the range from 900xc2x0 C. to 1100xc2x0 C. (typically around 1000xc2x0 C.) whereas the second temperature is in the range from 1600xc2x0 C. to 2100xc2x0 C.
By growing a polycrystalline silicon layer or a single crystal silicon layer by catalytic CVD using a catalyst of tungsten having formed a carbide on the surface thereof, for example, the maximum tungsten concentration in the polycrystalline silicon layer or the single crystal silicon layer can be controlled to 5xc3x971016 atoms/cm3 or less.
The carbide may be formed by stacking a carbide on the surface of the catalyst, instead of forming it by nitrifying the surface of the catalyst. For stacking the carbide, sputtering, metal-organic chemical vapor deposition (MOCVD), or other like method, for example, can be used.
In order to reduce the concentration of contained oxygen in addition to the concentration of the contained metal impurities in the polycrystalline silicon layer or single crystal silicon layer grown by catalytic CVD, it is effective to adjust the total pressure of the growth atmosphere to a value in the range from 1.33xc3x9710xe2x88x923 Pa to 4 Pa at least at the beginning of the growth, or adjust the partial pressure of oxygen and moisture in the growth atmosphere to a value in the range from 6.65xc3x9710xe2x88x9210 Pa to 2xc3x9710xe2x88x926 Pa at least at the beginning of the growth.
In the present invention, usable catalyst materials include, as simplex metals, tungsten (W) (3380xc2x0 C.), titanium (Ti) (1668xc2x0 C.), vanadium (V) (1905xc2x0 C.), zirconium (Zr) (1850xc2x0 C.), niobium (Nb) (2468xc2x0 C.), molybdenum (Mo) (2615xc2x0 C.), technetium (Tc) (2170xc2x0 C.), ruthenium (Ru) (2280xc2x0 C.), tantalum (Ta) (2998xc2x0 C.), rhenium (Re) (3160xc2x0 C.), osmium (Os) (3027xc2x0 C.), and iridium (Ir) (2443xc2x0 C.), for example. Temperatures after respective materials are their melting points. Usable materials other than simplex metals (alloys and compounds) include TaN, TaC, W2N, WN, NiW, NiWN, TiW, TiWN, and MoNi. As carbides, there are TaC, WC, TiWc.
In the present invention, growth temperature of a polycrystalline silicon layer or a single crystal silicon layer by catalytic CVD may be in the range from 200xc2x0 C. to 600xc2x0 C.
In the second and fifth aspects of the invention, the base layer for epitaxially growing the single crystal silicon layer on, i.e. the material layer in lattice alignment with single crystal silicon, may be made of single crystal silicon, or sapphire, spinel, or the like. The xe2x80x9csingle crystal siliconxe2x80x9d is used to involve those including sub-boundaries.
The growth method by catalytic CVD according to the invention can be used for manufacturing various types of semiconductor devices such as junction type FET and bipolar transistors in addition to thin-film transistors (TFT) that are MISFET, for example. Further, the method is applicable also to fabrication of diodes, capacitors and resistors, not limited to those transistors.
According to the invention having the above summarized configuration, since at least the surface of the catalyst is made up of a nitride or carbide, its component elements or metal impurities can be prevented from the catalyst when it is heated to a high temperature. Therefore, it is effectively prevented that these component elements or metal impurities are ingested into the growth layer while the a polycrystalline silicon layer or a single crystal silicon layer is grown by catalytic CVD.
Furthermore, by adjusting the total pressure of the growth atmosphere in the range from 1.33xc3x9710xe2x88x923 Pa to 4 Pa at least at the beginning of the growth, it is possible to maintain the partial pressure of oxygen and moisture in the growth atmosphere in the range from 6.65xc3x9710xe2x88x9210 Pa to 2xc3x9710xe2x88x926 Pa at least at the beginning of the growth and to remarkably reduce the ingestion of oxygen into the growth layer.
The above, and other, objects, features and advantage of the present invention will become readily apparent from the following detailed description thereof which is to be read in connection with the accompanying drawings.